.bg {
    width: 730px;
    height: 454px;
    margin: 20px auto;
    position: relative;
}

.bg img {
    width: 100%;
    height: 100%;
}

.left-btn {
    position: absolute;
    width: 20px;
    height: 40px;
    line-height: 40px;
    text-align: center;
    color: white;
    background-color: rgba(0, 0, 0, 0.5);
    left: 0;
    top: 50%;
    margin-top: -20px;
}

.right-btn {
    position: absolute;
    width: 20px;
    height: 40px;
    line-height: 40px;
    text-align: center;
    color: white;
    background-color: rgba(0, 0, 0, 0.5);
    right: 0;
    top: 50%;
    margin-top: -20px;
}

.bg ul {
    width: 60px;
    margin: 0;
    padding: 0;
    position: absolute;
    left: 50%;
    margin-left: -30px;
    height: 20px;
    list-style: none;
    bottom: 2px;
}

.bg ul li {
    width: 20px;
    text-align: center;
    height: 20px;
    line-height: 20px;
    float: left;
    border-radius: 50%;
    background-color: rgba(0, 0, 0, 0.5);
    color: white;
}